The present invention relates generally to apparatus for bonding electronic dies to a substrate and more specifically wherein the substrate is a ceramic carrier.
Inner lead bonders for bonding dies to the leads of a carrier tape can be performed with extreme accuracy since the positioning of the leads on the carrier tape is well controlled. A typical example is U.S. Pat. No. 4,526,646 to SUZUKI, ET AL. wherein the carrier tape 16 has its XY position adjusted with respect to the bond site. Die positioning fingers 43 adjust the .theta. and the X coordinate of the die with respect to Table 40. A die defect detection camera 81 is provided at the wafer pick up station. A die positional pattern detecting camera 83 detects positional deviation in the Y direction. A lead positional pattern detection camera 85 detects the position of the lead pattern at the bond site. The positional deviation of the lead pattern of the carrier tape 16 from the pattern of the die 51 detected by camera 83 is computed. Then, the XY table 31 is driven to adjust the tape carrier with respect to the bond head 10 in die 51 for the computed deviation.
The accuracy of the system like that in SUZUKI, ET AL. depends on the mechanical set up time to align the axis of cameras 83 and 85 to the system. Similarly, the accuracy of the alignment with respect to the moveable Tables 40 and 31 is not verified. This alignment between the camera axis and the magnification factor of the two cameras are not taken into account. Thus, the accuracy of the system is a function of the set up procedure.
A lead bonder to lead frame as in SUZUKI, ET AL. applies the die from below the lead frame. This same type of system cannot be used without modification wherein the die is to be placed on a substrate, for example a printed circuit board or a ceramic carrier. A typical example of a top mounting system is shown in U.S. Pat. No. 4,797,994 to MICHAUD, ET AL. The die is applied to a metallic substrate shown as a lead frame. An optical system is provided at the wafer for locating the die and indicating the presence of a substandard die at the pick up location. The details of the optical system is described in U.S. Pat. No. 4,823,394. The die is placed at a bond site which has received an application of epoxy or adhesive prior to being moved to the bond site location.
The quality of the bond of the electronic die to the substrate is a function of the pressure applied by the bonding or insertion tool. Some system applies a fixed pound force for a given amount of time. This does not take into account the deviation of the distance between the bond head and the surface which may result from misalignment of the bonding surface or nonplanarity of the bonding surface. One method which addresses this problem is described in U.S. Pat. No. 4,857,133 to MULLIN. Instead of applying a predetermined force by depressing the die onto the package at a predetermined separation distance, MULLIN brings the die and package into contact and records the position. Then they are separated and paste is applied to the package. Finally, the die and the package are repositioned at the predetermined position from the recorded position.
This mechanical procedure of determining the alignment of depth along the Z axis takes a considerable amount of time and it generally cannot be used in mass production. An apparatus for non-contact sensing, and controlling of spacing between a depositing tip and selected depositing location on a substrate, is described in U.S. Pat. No. 4,762,578 to BURGIN ET AL. The sensor is a laser system and the depositing tip may be a paste nozzle or a nozzle which picks up dies and places them at the bonding site.
Another problem with die bonding machines to a substrate versus a lead frame is that the pattern for the bonding paste must be developed for each unique sized die. Generally the paste is applied as a plurality of dots resembling a shower head. To improve upon this process, a starfish pattern has been used as described in U.S. Pat. No. 4,803,124 to KUNZ. Using either method, the pattern is specifically designed for the size of the die. This is not a problem on large runs, but for smaller runs through an automatic bonding machine, there is a lot of down time in developing the appropriate characteristics of the bonding paste pattern. As with the previously discussed patents, the bonding material is applied to the substrate or carrier surface generally prior to the bonding location. There is no pre-inspection of the bonding paste layer or pattern to determine whether it will produce an appropriate bond. Post-bonding inspection, if any, would not prevent wasting a die and a substrate if preinspected.
Another problem which varies the accuracy of alignment in bonding the die to a substrate is that the substrates are generally carried on boats versus lead frames which are part of the carrier. The boat is adjusted to its position at the bonding location and the substrate may shift with respect to the boat prior to the bonding process. This substantially affects the accuracy of alignment of the die to the substrate at the desired bond site.
Thus the present invention is directed to a bonding system which minimizes the set-up time while maximizes the accuracy of alignment.
Another object of the present invention is to provide a bonding system wherein the alignment of the various elements of the system are determined in a set-up stage and mathematically correlated to reduce the set-up time.
Still another object of the present invention is to provide a bonding system which monitors and compensates for X, Y, Z and .theta. during bonding.
Still an even further object of the present invention is to provide a bonding system which monitors the quality of the bonding paste pattern prior to bonding.
An even further object of the present invention is to provide a bonding system wherein the monitoring and correction of the orientation of the die to the substrate is performed after paste is applied to the die.
Still an even further object of the present invention is to provide a bonding system where not only the height of the bonding surface with respect to a reference is determined, but also the planarity of the bonding surface.
Still an even further object of the present invention is to provide a bonding system which has inspection of the bonding surface prior to bonding and of the bond after bonding.
Still an even further object of the present invention is to provide a bonding system which includes inspection of the die at the wafer before being picked up and inspection of the substrate prior to bonding so as to maximize acceptable bonded product.
Still an even further object of the present invention is to provide a bonding system wherein the position of the bond site prior to bonding is verified even after initial adjustments.
These and other objects are attained by an aligning and positioning system for a bonder including a first or bond sensor for sensing the X, Y and .theta. coordinates of a substrate at the bonding location and a second or die sensor for sensing the X, Y and .theta. coordinates of the die on a die or second transport which transports the die from a supply location to the bond site on the substrate. A controller correlates the sensed coordinates from the first and second sensors and adjusts the X, Y and .theta. of the die and the substrate relative to each other to place the die at a preselected bond site on the substrate. The system mathematically compensates for any misalignment. The Z coordinate of the surface of the substrate, as well as its planarity, before bonding and the Z coordinate and planarity of the die after bonding may be sensed by a third optical sensor. The first and second sensors may be optical sensors. The X and Y coordinates of the substrate are adjusted and the .theta. coordinate of the die on the die transport is adjusted. The X, Y and .theta. of the die after bonding is the also measured.
A pattern recognition system is used in the first and second sensors to determine the coordinates as well as quality control. First sensor can monitor the quality of the substrate prior to bonding. This will include the planarity as well as the conductive or paste pattern if any on the substrate. The first sensor may also monitor the quality of the bond after bonding. The second sensor, which will monitor the quality of the layer of bonding paste on the die, if applied prior to bonding.
In a calibration phase, the controller correlates the X, Y and .theta. coordinates of the substrate or first transport and the first sensor. It also correlates the X, Y and .theta. coordinates of the die transport, which carries the die to the bond site, with the second sensor. The magnification factor of the two sensors, which are generally cameras, are also correlated. This mathematically compensates this system for all factors involved such that there is an active correlation of the sensed coordinates of the die and the substrate by the sensor and positioning system. The position of the die on the die transport is determined after pasting and before bonding. The desired bond site is also verified after the X, Y adjustments of the substrate and the .theta. adjustment of the die to assure there has not been any further misalignment prior to bonding. If need, another X, Y and .theta. adjustment is performed.
The method of calibration includes determining and recording the X, Y and .theta. offset between the first and second sensors, and also determining and recording the .theta. offset between the first sensor and the first transport of the substrate. The center-line of the first sensor and of the substrate transport at the bonding station are aligned. Similarly, the center-line of the second transport for the die and the first transport for the substrate are aligned to the bonding location. The center of rotation of the die transport in the coordinate system of the second sensor is determined and recorded.
The first and second sensors are cameras and their magnification factor is determined and recorded. The magnification factor is determined by selecting a pair of reference points a known distance apart for the first sensor and the second sensor and determining the distance between the pair of reference points in the field of view of the camera. These reference points may be at two corners of a reference object.
The .theta. offset between the first sensor and the first transport is determined by selecting a reference point on the first transport, activating the first transport to move the reference point in the X and Y direction of the first transport, and determine the change of the position of the reference point in coordinate system of the first sensor. Similarly, the reference point may be one or more of the corners of a referenced object.
The .theta. offset between the die transport and the substrate transport is determined by positioning a reference object on the substrate transport at the bonding location and determining the .theta. orientation of the reference object in the coordinate system of the first sensor. Next, activating the die transport to pick up the reference object and to move the reference object to the second sensor and determine the .theta. orientation of the reference object in the coordinate system of the second sensor. Determining the .theta. orientation of the reference object in either of the sensors is performed by determining the position of at least three of the four corners of the reference object. The center of rotation of the second or die transport in the coordinate system of the second sensor is determined by rotating the die transport through a predetermined angle at the second sensor.
The general method of bonding includes determining the position of the die with respect to a bonding head or die transport, determining the position of a substrate at a bonding location, rotating the bonding head to position the die in the desired .theta. alignment with the substrate, positioning the substrate in X and Y coordinates to position a desired bond site at the bonding location, moving the bonding head with the die to the bonding location, and bonding the die to the bond site. Additionally, the position of the bonding site at the bonding location is determined after position of substrate and before moving the bonding head to assure alignment.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.